Cluster Preferences Specification

A preference is a hint to Solr on how to sort nodes based on their utilization. The default cluster preference is to sort by the total number of Solr cores (or replicas) hosted by a node. Therefore, by default, when selecting a node to which to add a replica, Solr can apply the preferences and choose the node with the fewest cores.

More than one preference can be added to break ties. For example, we may choose to use free disk space to break ties if the number of cores on two nodes is the same. The node with the higher free disk space can be chosen as the target of the cluster operation.

Each preference takes the following form:

{"<sort_order>":"<sort_param>","precision":"<precision_val>"}

sort_order

The value can be either maximize or minimize. Choose minimize to sort the nodes with least value as the least loaded. For example, {"minimize":"cores"} sorts the nodes with the least number of cores as the least loaded node. A sort order such as {"maximize":"freedisk"} sorts the nodes with maximum free disk space as the least loaded node.

The objective of the system is to make every node the least loaded. So, in case of a MOVEREPLICA operation, it usually targets the most loaded node and takes load off of it. In a sort of more loaded to less loaded, minimize is akin to sorting in descending order and maximize is akin to sorting in ascending order.

This is a required parameter.

sort_param

One and only one of the following supported parameters must be specified:

cores: The number of total Solr cores on a node.

freedisk: The amount of free disk space for Solr’s data home directory. This is always in gigabytes.

sysLoadAvg: The system load average on a node as reported by the Metrics API under the key solr.jvm/os.systemLoadAverage. This is always a double value between 0 and 1 and the higher the value, the more loaded the node is.

heapUsage: The heap usage of a node as reported by the Metrics API under the key solr.jvm/memory.heap.usage. This is always a double value between 0 and 1 and the higher the value, the more loaded the node is.

precision

Precision tells the system the minimum (absolute) difference between 2 values to treat them as distinct values.

For example, a precision of 10 for freedisk means that two nodes whose free disk space is within 10GB of each other should be treated as equal for the purpose of sorting. This helps create ties without which specifying multiple preferences is not useful. This is an optional parameter whose value must be a positive integer. The maximum value of precision must be less than the maximum value of the sort_value, if any.

Examples of Cluster Preferences

Default Preferences

The following shows the default cluster preferences. This is applied automatically by Solr when no explicit cluster preferences have been set using the Autoscaling API.

[{"minimize":"cores"}]

Minimize Cores; Maximize Free Disk

In this example, we want to minimize the number of Solr cores and in case of a tie, maximize the amount of free disk space on each node.

[{"minimize":"cores"},{"maximize":"freedisk"}]

Add Precision to Free Disk; Minimize System Load

In this example, we add a precision to the freedisk parameter so that nodes with free disk space within 10GB of each other are considered equal. In such a case, the tie is broken by minimizing sysLoadAvg.

Policy Specification

A policy is a hard rule to be satisfied by each node. If a node does not satisfy the rule then it is called a violation. Solr ensures that the number of violations are minimized while invoking any cluster management operations.

Policy Rule Structure

Rule Types

Policy rules can be either global or per-collection:

Global rules constrain the number of cores per node or node group. This type of rule applies to cores from all collections hosted on the specified node(s). As a result, collection-specific policies, which are associated with individual collections, may not contain global rules.

Per-collection rules constrain the number of replicas per node or node group.

Except for node, the attributes above cause selected nodes to be partitioned into node groups. A node group is referred to as a "bucket". Those attributes usable with the #EQUAL directive may define buckets either via the special function #EACH or an array["value1", …​] (a subset of all possible values); in both cases, each node is placed in the bucket corresponding to the matching attribute value.

The node attribute always places each selected node into its own bucket, regardless of the attribute value’s form (#ANY, node-name, or ["node1-name", …​]).

Replica and core count constraints, described below, are evaluated against the total number in each bucket.

Core Count Constraint

The cores attribute value can be specified in one of the following forms:

Specifying Replica and Core Count Constraints

Replica count constraints ("replica":"…​") and core count constraints ("cores":"…​") allow specification of acceptable counts for replicas (cores tied to a collection) and cores (regardless of the collection to which they belong), respectively.

You can specify one of the following as the value of a replica and cores policy rule attribute:

an exact integer (e.g., 2)

an exclusive lower integer bound (e.g., >0)

an exclusive upper integer bound (e.g., <3)

a decimal value, interpreted as an acceptable range of core counts, from the floor of the value to the ceiling of the value, with the system preferring the rounded value (e.g., 1.6: 1 or 2 is acceptable, and 2 is preferred)

a range of acceptable replica/core counts, as inclusive lower and upper integer bounds separated by a hyphen (e.g., 3-5)

a percentage (e.g., 33%), which is multiplied at runtime either by the number of selected replicas (for a replica constraint) or the number of cores in the cluster (for a cores constraint). This value is then interpreted as described above for a literal decimal value.

Note

Using an exact integer value for count constraints is of limited utility, since collection or cluster changes could quickly invalidate them. For example, attempting to add a third replica to each shard of a collection on a two-node cluster with policy rule {"replica":1, "shard":"#EACH", "node":"#ANY"} would cause a violation, since at least one node would have to host more than one replica. Percentage rules are less brittle. Rewriting the rule as {"replica":"50%", "shard":"#EACH", "node":"#ANY"} eliminates the violation: 50% of 3 replicas = 1.5 replicas per node, meaning that it’s acceptable for a node to host either one or two replicas of each shard.

Policy Rule Attributes

Rule Strictness

This attribute is usable in all rules:

strict

An optional boolean value. The default is true. If true, the rule must be satisfied; if the rule is not satisfied, the resulting violation will cause the cluster management operation to fail. If false, Solr tries to satisfy the rule on a best effort basis, but if no node can satisfy the rule, the cluster management operation will not fail, and any node may be chosen. If multiple rules declared to be strict:false can not be satisfied by some nodes, then a node will be chosen such that the number of such violations is minimized.

Per-collection Rule Attributes

The name of the collection to which the policy rule should apply. If omitted, the rule applies to all collections. This attribute is optional.

shard

The name of the shard to which the policy rule should apply. If omitted, the rule is applied for all shards in the collection. It supports the special function #EACH which means that the rule is applied for each shard in the collection.

type

The type of the replica to which the policy rule should apply. If omitted, the rule is applied for all replica types of this collection/shard. The allowed values are NRT, TLOG and PULL

The free disk space in gigabytes of the node. This must be a positive 64-bit integer value, or a percentage. If a percentage is specified, either an upper or lower bound may also be specified using the < or > operators, respectively, e.g., >50%, <25%.

host

The host name of the node.

sysLoadAvg

The system load average of the node as reported by the Metrics API under the key solr.jvm/os.systemLoadAverage. This is floating point value between 0 and 1.

heapUsage

The heap usage of the node as reported by the Metrics API under the key solr.jvm/memory.heap.usage. This is floating point value between 0 and 1.

nodeRole

The role of the node. The only supported value currently is overseer.

ip_1, ip_2, ip_3, ip_4

The least significant to most significant segments of IP address. For example, for an IP address 192.168.1.2, "ip_1":"2", "ip_2":"1", "ip_3":"168", "ip_4":"192". The array operator may be used in any of these attributes' values.

Any arbitrary metric. For example, metrics:solr.node:CONTAINER.fs.totalSpace. Refer to the key parameter in the Metrics API section.

diskType

The type of disk drive being used for Solr’s coreRootDirectory. The only two supported values are rotational and ssd. Refer to coreRootDirectory parameter in the Solr.xml Parameters section. The ! (not) operator or the array operator may be used in this attribute’s value.

Its value is fetched from the Metrics API with the key named solr.node:CONTAINER.fs.coreRoot.spins. The disk type is auto-detected by Lucene using various heuristics and it is not guaranteed to be correct across all platforms or operating systems. Refer to the Dynamic defaults for ConcurrentMergeScheduler section for more details.

Policy Operators

Each attribute in the policy may specify one of the following operators along with the value.

No operator means equality

<: Less than

>: Greater than

!: Not

Range operator (-): a value such as "3-5" means a value between 3 to 5 (inclusive). This is only supported in the replica and cores attributes.

Array operator []: e.g., sysprop.zone= ["east","west","apac"]. This is equivalent to having multiple rules with each of these values. This can be used in the following attributes:

Special Functions

#ANY: Applies to the node attribute only. This means the rule applies to any node.

#ALL: Applies to the replica attribute only. This means all replicas that meet the rule condition.

#EACH: Applies to the shard attribute (meaning the rule should be evaluated separately for each shard), and to the attributes used to define the buckets for the #EQUAL function (meaning all possible values for the bucket-defining attribute).

#EQUAL: Applies to the replica and cores attributes only. This means an equal number of replicas/cores in each bucket. The buckets can be defined using the below attributes with a value that can either be #EACH or a list specified with the array operator ([]):

Examples of Policy Rules

Limit Replica Placement

Do not place more than one replica of the same shard on the same node. The rule is evaluated separately for each shard in each collection. The rule is applied to any node.

{"replica":"<2","shard":"#EACH","node":"#ANY"}

Limit Cores per Node

Do not place more than 10 cores in any node. This rule can only be added to the cluster policy because it is a global rule.

{"cores":"<10","node":"#ANY"}

Place Replicas Based on Port

Place exactly 1 replica of each shard of collection xyz on a node running on port 8983.

{"replica":1,"shard":"#EACH","collection":"xyz","port":"8983"}

Place Replicas Based on a System Property

Place all replicas on nodes with system property availability_zone=us-east-1a.

{"replica":"#ALL","sysprop.availability_zone":"us-east-1a"}

Use Percentage

Place a maximum of (roughly) a third of the replicas of each shard in any node. In the following example, the value of replica is computed in real time as a percentage of the replicas of each shard of each collection:

{"replica":"33%","shard":"#EACH","node":"#ANY"}

If the number of replicas in a shard is 2, 33% of 2 = 0.66. This means a node may have a maximum of 1 and a minimum of 0 replicas of each shard.

It is possible to get the same effect by hard coding the value of replica as a decimal value:

Place Replicas Based on Free Disk

Keep all replicas in nodes where freedisk percentage is greater than 50%.

{"replica":"#ALL","freedisk":">50%"}

Try to Place Replicas Based on Free Disk

When possible, place all replicas in nodes where freedisk is greater than 500GB. Here we use the strict attribute to signal that this rule is to be honored on a best effort basis.

{"replica":"#ALL","freedisk":">500","strict":false}

Place All Replicas of Type TLOG on Nodes with SSD Drives

{"replica":"#ALL","type":"TLOG","diskType":"ssd"}

Place All Replicas of Type PULL on Nodes with Rotational Disk Drives

{"replica":"#ALL","type":"PULL","diskType":"rotational"}

Defining Collection-Specific Policies

By default, the cluster policy, if it exists, is used automatically for all collections in the cluster. However, we can create named policies that can be attached to a collection at the time of its creation by specifying the policy name along with a policy parameter.

When a collection-specific policy is used, the rules in that policy are appended to the rules in the cluster policy and the combination of both are used. Therefore, it is recommended that you do not add rules to collection-specific policy that conflict with the ones in the cluster policy. Doing so will disqualify all nodes in the cluster from matching all criteria and make the policy useless.

It is possible to override rules specified in the cluster policy using collection-specific policy. For example, if a rule {replica:'<3', node:'#ANY'} is present in the cluster policy and the collection-specific policy has a rule {replica:'<4', node:'#ANY'}, the cluster policy is ignored in favor of the collection policy.

Also, if maxShardsPerNode is specified during the time of collection creation, then both maxShardsPerNode and the policy rules must be satisfied.

Some attributes such as cores can only be used in the cluster policy. See the section Policy Rule Attributes for details.

To create a new named policy, use the set-policy API. Once you have a named policy, you can specify the policy=<policy_name> parameter to the CREATE command of the Collection API:

The above CREATE collection command will associate a policy named policy1 with the collection named coll1. Only a single policy may be associated with a collection.

Example: Manual Collection Creation with a Policy

The starting state for this example is a Solr cluster with 3 nodes: "nodeA", "nodeB", and "nodeC". An existing 2-shard FirstCollection with a replicationFactor of 1 has one replica on "nodeB" and one on "nodeC". The default Autoscaling preferences are in effect:

[{"minimize":"cores"}]

The configured policy rule allows at most 1 core per node:

[{"cores":"<2","node":"#ANY"}]

We now issue a CREATE command for a SecondCollection with two shards and a replicationFactor of 1:

For each of the two replicas to be created, each Solr node is tested, in order from least to most loaded: would all policy rules be satisfied if a replica were placed there using an ADDREPLICA sub-command?

ADDREPLICA for shard1: According to the Autoscaling preferences, the least loaded node is the one with the fewest cores: "nodeA", because it hosts no cores, while the other two nodes each host one core. The test to place a replica here succeeds, because doing so causes no policy violations, since the core count after adding the replica would not exceed the configured maximum of 1. Because "nodeA" can host the first shard’s replica, Solr skips testing of the other two nodes.

ADDREPLICA for shard2: After placing the shard1 replica, all nodes would be equally loaded, since each would have one core. The test to place the shard2 replica fails on each node, because placement would push the node over its maximum core count. This causes a policy violation.

Since there is no node that can host a replica for shard2 without causing a violation, the overall CREATE command fails. Let’s try again after increasing the maximum core count on all nodes to 2:

[{"cores":"<3","node":"#ANY"}]

After re-issuing the SecondCollection CREATE command, the replica for shard1 will be placed on "nodeA": it’s least loaded, so is tested first, and no policy violation will result from placement there. The shard2 replica could be placed on any of the 3 nodes, since they’re all equally loaded, and the chosen node will remain below its maximum core count after placement. The CREATE command succeeds.